Process for the coating of metal

ABSTRACT

AN IMPROVED PROCESS FOR COATING METAL WHEREIN A PHOSPHATE OR OXALATE CONVERSION COATING COMPOSITION IS APPLIED TO AN ALUMINUM, ZINC OR FERROUS METAL SURFACE TO FORM A SUBSTANTIALLY DRY, UNIFORM PHOSPHATE OR OXALATE CONVERSION COATING, A PORTION OF WHICH IS WATER-SOLUBLE. AN ORGANIC CONTAINING REDUCING AND/OR ESTERIFYING FIXING OR IMMOBILIZING COATING COMPOSITION CONTAINING HAXAVALENT CHROMIUM IS THEN APPLIED WHICH REACTSOUT THE WATERSOLUBLE PORTION OF THE CONVERSION COATING TO PRODUCE WATER-INSOLUBLE PHOSPHATE OR OXALATES AND FORM A SUBSTANTIALLY DRY, UNIFORM COMPOSITE COATING. THIS PROCESS ELIMINATES THE NEED FOR RINSING PROCEDURES.

United States Patent O 3,706,604 PROCESS FOR THE COATING OF METAL Elmer H. Plaxton, Bloomfield Hills, Mich., assignor to Oxy Metal Finishing Corporation, Warren, Ohio No Drawing. Continuation-impart of application Ser. No.

789,005, Dec. 31, 1968, which is a continuation-in-part of applications Ser. No. 378,946, June 29, 1964, and

Ser. No. 378,982, June 29, 1964. This application Jan.

20, 1971, Ser. No. 108,200

Int. Cl. C231? 7/10, 7/14, 7/18 US. Cl. 1486.16 8 Claims ABSTRACT OF THE DISCLOSURE An improved process for coating metal wherein a phosphate or oxalate conversion coating composition is applied to an aluminum, zinc or ferrous metal surface to form a substantially dry, uniform phosphate or oxalate conversion coating, a portion of which is water-soluble. An organic containing reducing and/or esterifying fixing or immobilizing coating composition containing hexavalent chromium is then applied which reacts out the watersoluble portion of the conversion coating to produce water-insoluble phosphate or oxalates and form a substantially dry, uniform composite coating. This process eliminates the need for rinsing procedures.

This application is a continuation-in-part application of Ser. No. 789,005, filed Dec. 31, 1968 now Pat. No. 3,573,997 issued Apr. 6, 1971, which is in turn a continuation-in-part of application Ser. Nos. 378,946 and 378,982, filed June 29, 1964 both now abandoned.

This invention relates to an improved process for coating metal surfaces and more particularly relates to improvements in the process for applying a protective coating to metallic surfaces such as zinc, iron and aluminum.

The desirability of providing metal surfaces, and particularly metal surfaces which are predominantly zinc, iron or aluminum, with a protective coating, such as phosphate coatings, has long been recognized. Such coatings are particularly desirable for preventing staining or corrosion of the metal surfaces, or to provide a base for paint or similar protective coatings.

In the past, particularly with surfaces which are predominantly zinc, difficulties have sometimes been encountered in obtaining a sufiiciently heavy'or uniform coating to provide the desired protection for the surface. In other instances, the phosphate coating obtained has been sufiiciently loose or non-adherent that it has been an unsatisfactory base for paint or other protective coatings. Often, these difficulties have increased or occurred more frequently when the coating has been applied to the surface of a moving strip or sheet of metal. In such instances, because of the speed at which the material is moving, the application of the coating material and the formation of the desired protective coating should be accomplished in a relatively short period of time. Moreover, it sometimes happens that in installations which treat predominantly zinc surfaces, such as those resulting from electro-plating zinc or hot-dip galvanizing, it is also desirable to treat surfaces which are predominantly iron or aluminum. Heretofore, it has not generally been possible to treat such a variety of surfaces to provide a satisfactory protective or paint-base coating using the compositions and application techniques which have been available.

Additionally, with processes and compositions which have been available for such applications, close controls have been dictated and frequently one or more rinses of the coated surfaces were employed to obtain acceptable results.

3,706,604 Patented Dec. 19, 1972 It is, therefore, an object of the present invention to provide novel coating compositions of the reactive phosphate type which may be applied to metal surfaces which are predominantly zinc, iron or aluminum, to provide an effective protective or paint-base coating.

Another object of the present invention is to provide a novel method whereby the coating compositions of the present invention may be easily and efficiently applied to metal surfaces of zinc, iron or aluminum, which surfaces are in the form of sheets and/or strips and/or other shapes which may be coated using various spraying techniques.

A urther object of the present invention is to provide novel compositions and methods for applying such compositions which are particularly adapted for forming paint-base coatings on zinc surfaces, produced by hot-dip galvanizing.

A yet further object of the present invention is to provide a novel process for coating metallic surfaces and particularly, zinc surfaces, which process does not require close controls and rinses.

These and other objects of the present invention will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for coating a metal surface which comprises applying to the metal surface to be coated a phosphate or oxalate conversion coating composition, effecting formation of a substantially dry, uniform conversion coating on the metal surface, a portion of which is watersoluble, applying onto the thus-formed coating on organiccontaining reducing and/or esterifying immobilizing or fixing composition containing additionally a hexavalent chromium stabilizing of passivating agent, reacting out the water-soluble portion of the previously formed conversion coating to produce Water-insoluble phosphates or oxalates and form a substantially dry, uniform composite coating on the metal surface. It Has been found that the treatment of zinc, iron and aluminum surfaces with the compositions and process of the present invention obviates the requirement for a rinse following the application thereto of a conversion coating composition, and renders the metal substrates particularly receptive to subsequently applied paints.

Preferably, in the present process, the conversion coating is applied using spray techniques and the metal surface to which it is applied is desirably at a temperature of at least about degrees Fahrenheit. Moreover, in effecting the formation of substantially dry, uniform coatings, this may be done by a separate step, e.g., heating after the application of the coating materials, or this may take place substantially simultaneously with the application of the coating material Where the metal surface is sufiiciently hot, e.g., at a temperature in excess of about 175 degrees Fahrenheit. Accordingly, both preheating and/or postheating of the metal surface may be used to effect formation of the substantially dry, uniform coating.

More specifically, in the process of the present invention, the preferred conversion coating composition is of the phosphate type in which the principal source of the phosphate ion is a metal phosphate wherein the metal has a valence of at least two. Typically, such conversion coating compositions are aqueous acidic solutions of a metal phosphate such zinc phosphate, ferrous phosphate, ferric phosphate, nickel phosphate, manganese phosphate, calcium phosphate, chromium phosphate, cobalt phosphate, aluminum phosphate, cadmium phosphate and the like. Additionally, mixed metallic phosphates such as zinc nickel phosphate, zinc iron phosphate, zinc calcium phosphate, iron manganese phosphate and the like may also be used. If desired, instead of the metal phosphates, the conversion coating solutions may contain phosphoric acid as the principal source of phosphate. In such compositions, as well as those based on metal phosphate, particularly where these also contain iron, it is generally desirable to have an auxiliary acid, other than phosphoric acid, which acid may be either an organic or inorganic acid. Exemplary of the auxiliary acids are nitric acid, acrylic acid, methacrylic acid, polyacrylic acid, adipic acid, acetic acid, hydroxyacetic acid, gluconic acid, fumaric acid, malic acid, maleic acid, malonic acid, lactic acid, itaconic acid, tartaric acid, ethylenediamine tetraacetic acid, critic acid, hydroxymethyl phosphonic acid, oxalic acid and the like. These auxiliary acids are typically present in amounts to provide a ratio of phosphate to the auxiliary acid in the range of from about 100:1 to about 2:1, with amounts suflicient to provide a ratio of from about :1 to about 4:1 being preferred. It will be appreciated that, in many instances, often depending upon the mode of application of the conversion coating composition, the compositions may contain the auxiliary acid in amounts both less than and greater than those which have been indicated hereinabove, to obtain comparable results.

Typically, the conversion coating compositions contain from about 0.1 to about 10.0 percent by weight phosphate, amounts between the range of from about 0.3 to about 6.0 percent by weight phosphate being preferred. In some instances, often depending upon the mode of application, amounts both less than 0.1 percent and greater than 10 percent may be used to obtain good results. As is known in the art, these and other phosphates provide the protective phosphate conversion coating on the metal surfaces being treated.

In addition to these materials, other materials may also be included in the conversion coating composition, e.g., materials which act as accelerators to reduce the time required to obtain a uniform, substantially continuous coating. Such materials include nitrates, nitrites, halides such as bromides, chlorates, bromates, sulfites, perchlorates, iodates, peroxides, periodates, permanganates, organic nitro compounds such as m-nitrobenzene sulfonate, nitro guanidine, nitromethane, nitroethane, 1-nitropropane, nitrobenzene, O-nitro benzaldehyde, p-nitrophenol, p-nitro aniline, p-nitro chlorobenzene, picric acid and the like. Other additives which may be used in the composition are those which increase the etching tendency of the solution such as the so-called complex fluorides including fiuosilicates, fluoborates, fluotitanates, fluostannates, fluozirconates, fluovanadates and the like.

Inasmuch as the conversion coating composition is desirably in the form of an aqueous solution, the above and other additives are preferably added to the composition in a form which is readily dispersible in the aqueous solution. Many water dispersible forms of these compounds may be used provided they have no adverse effect on the coating composition, the metal substrate, or the coatings subsequently produced or applied. Typically, such additives may be present in the conversion coating composition in amounts within the range of from about 0.01 to about 6.0 percent by weight, and desirably in amounts within the range of from about 0.02 to about 4.0 percent by weight, although in many instances both lesser and greater amounts may be used.

In many instances, the oxalate radical may be substituted for the phosphate in these compositions to obtain comparable results. Exemplary of this is a conversion coating composition solution based on ferric oxalate, rather than the phosphate. Additionally, adjuvantsother than those which have been indicated hereinabove may also be included in the compositions where particular properties and/or characteristics of the coating are desired. Typically, these conversion coating compositions are utilized at a pH within the range of from about 0.7 to about 3.5 and the compositions are applied in such a manner so as to obtain a coating weight of phosphate ions of from about 10 to about 250 milligrams per square 4 foot of metal surface being treated. Desirably, the coatings obtained contain phosphate in the range of from about 20 to about milligrams per square foot. Here again both lesser and greater amounts may be used.

The conversion coating compositions are desirably applied to metal surfaces which are preferably at a temperature of about degrees Fahrenheit, or higher, immediately before impingement of the phosphate composition. Typically, the temperatures are within the range of from about 200 to about 600 degrees Fahrenheit, with temperatures in the range of from about 275 to about 425 degrees Fahrenheit being most preferred. Although various metal surfaces may be treated in this manner, e.g., surfaces which are predominantly zinc, iron or aluminum, excellent results have been obtained when coating zinc surfaces and, in particular, zinc surfaces obtained by hot-dip galvanizing. In such preferred applications, the process of the present invention may be carried out immediately following the hot-dip galvanizing operation, while the surfaces are still hot from the galvanizing bath. In this manner, the retained heat of the metal from the hot-dip galvanizing operation is utilized so that the application of additional heat to the surfaces may not be necessary. It will be appreciated, of course, that where the metal surface to he treated is not already hot from some previous processing step, preheating of the surface prior to the application of the conversion coating composition, may be carried out in many convenient ways to obtain the desired temperatures that have been indicated hereinabove.

The phosphate conversion coating compositions employed in the present process may be applied to the metal surfaces by various means, for example by spraying, immersion, flooding, roll-on and the like. Of these, various spraying techniques are generally utilized with the preferred method of application being by misting. In this latter technique, the coating solution to be applied is subjected to atomization so as to obtain from very finely divided liquid particles which are deposited on the metal surface in such a manner as to result in substantially no liquid run-ofl from the surface and a dry, substantially uniform coating is very quickly obtained, as the liquid components of the composition are evaporated by the heat of the metal surface. Typically, this will be effected by steam or other gas atomization. In such misting, gasatomized particles, having a size in the range of from about 15 to about 350 microns, of the aqueous coating solution are directed toward a heated metal surface so as to form a coating on the surface by inter-reaction with the heated surface. The size of the particles, and the quantity and number of successive applications of these particles to the surface, are all controlled in order that the particles of coating composition deposited on the surface remain in substantially the loci of their original impacts, and the surface is uniformly coated. Generally, it has been found that by utilizing such misting techniques for applying the coating compositions of the present invention, the time required to obtain a substantially dry, uniform coating on the surface being treated is greatly reduced and more eflicient use of the coating composition is made with substantially no waste or run-01f from the surface. Accordingly, hereinafter, in referring to the application of the conversion coating compositions to the surfaces to be treated, primary reference will be made to the use of misting techniques to accomplish this application. This is not, however, to be taken as a limitation of the application techniques which may be used, as other and perhaps more conventional methods have been used in the process of the present invention to obtain excellent results.

It will be appreciated by those skilled in the art that the application of the phosphate conversion coatings by misting to the heated zinc surfaces may be accomplished with many different types of equipment, depending upon the particular circumstances involved. For example, where the metal being treated is in the form of strips or large sheets, these may be passed through one or more stationary spraying or misting zones, wherein the size and number of the spraying zones, the concentration of the phosphate conversion coating solution and the particle size of the atomized solution may all be varied so as to obtain the desired coating weight, within the ranges indicated hereinabove. Alternatively, and particularly in treating smaller workpieces, hand spraying may be employed wherein multiple passes of the spray over the surface to be treated, for varying periods of time, may also be utilized to control and obtain the desired coating weights. These and other application methods are readily apparent to those having skill in the art and the particular techniques utilized will, of course, depend upon the specific factors involved. In general, however, it has been found that the coating Weights applied can be controlled over the greatest range by variations in the concentration of the conversion coating solution.

After the formation of a substantially uniform phosphate conversion coating on the metal surface has been effected, a portion of which coating is water-soluble, an organic reducing and/or esterifying fixing composition is then applied to the thus-formed coating. As has been indicated hereinabove, due to the heat of the metal surface in the preferred method, the formation of a substantially dry, uniform phosphate or oxalate conversion coating takes place very quickly and in many instances, substantially simultaneously with the application of the phosphate or oxalate conversion coating material. Thus, the application of the fixed coating composition can be effected substantially immediately after the application of the conversion coating material, with no rinsing or curing of the phosphate or oxalate coating.

The fixing coating composition, as previously stated, contains organic reducing and/or esterifying materials which are reactive with the water-soluble portion of the previously formed conversion coating, and also contains hexavalent chromium. These materials are preferably dissolved or dispersed in an aqueous media. In referring to fixing or immobilizing the conversion coating, it is meant that the second coating material applied reacts with the water-soluble portion of the previously applied conversion coating to make it adherent to a subsequently applied paint or similar protective coating and/or make it provide an improved barrier between the metal substrate and materials with which it may come in contact which are likely to be corrosive or to lift the paint from the surfacefsuch as water, thereby immobilizing or fixing and stabilizing the conversion coating on the metal surface. I

The reactions by which this fixing and stabilizing is accomplished involve esterification and/or reduction of the Water-soluble portion of the phosphate or oxalate conversion coating with a concurrent stabilization or passivation by the hexavalent chromium present in the treating composition. An immediate advantage of the compositions and process of the present invention resides in the fact that no rinsing of the conversion coating is required and the treated metal is extremely receptive to the application and retention of paints.

The organic compounds which may be included in the fixing composition to effect these reactions include alcohols such as butanols, pentanols, hexanols, tetrahydrofurfuryl alcohol and the like; diols such as butene diols, butane diols propylene glycols, ethylene glycol, resorcinol and the like; polyols such as those resulting from the condensation of ethylene oxide with propylene glycol and propylene oxide and the like; formaldehyde donors such as dimethylol urea, methylol dimethyl hydantoin, and the like; alkanolamines such as triethanolamine, tripropanolamine, hydroxyethylamine and the like; resin intermediates such as hexamethylol-melamine, urea-formaldehyde and the like; hydroquinone, pyro catechol, pyrogallol, p-methylam inophenyl sulfate, N-phydroxyphenylglycine and the like. With respect to the 6 alcohol, diols and polyols, these typically will contain from about 3 to about 20 carbon atoms in a straight or branched chain or as an aromatic group.

In some instances, it has been found to be desirable to include a colloidal silica solution in the fixing composition. The addition of colloidal silica appears to enhance the quality of the coating obtained. When employed, the colloidal silica is present in an amount from about 0.1 to about 0.5 percent by weight, based on the weight of the fixing composition, and preferably in an amount of flom about 0.2 to about 0.4 percent by Weight.

As with the phosphate or oxalate conversion coating material, the concentration of the esterifying and/or reducing components of the fixing composition may be varied, typical concentrations being within the range of from about 0.01 to about 6 percent by weight of the total composition, with amounts within the range of from about 0.05 to about 1 percent being preferred. Similarly, the amount of the immobilizing component deposited on the surface being treated may also be varied amounts within the range of from about 1 to about milligrams per square foot being typical with amounts Within the range of from about 15 to about 50 milligrams per square foot being preferred. These concentrations and coating weights are exemplary of those which may be used, as both greater and lesser amounts give good results in many instances.

The hexavalent chromium employed in the present compositions is derived from chromates and dichromates of metals having a valence of at least two. Typical of such materials which may be used are zinc chromate and dichromate, aluminum chromate and dichromate, calcium chromate and dichromate, ferric chromate and dichromate, ferrous chromate and dichromate, cobalt chromate and dichromate, nickel chromate and dichromate, chromium chromate and dichromate and the like, as well as mixtures thereof. Additionally, in many instances, the hexavalent chromium may be added to the treating composition as chromic acid (CrO preferably in the form of an aqueous solution. The chromic acid may be used as such or, if desired, combined with other materials such as aluminum oxide, colloidal silica, Water glass, ammonical zinc oxide, silica-ammonical zinc oxide and the like. Important to the compositions of the present invention is the presence of chromium as Cr+ with substantially no (Ir- The concentration of the hexavalent chromium component of the composition may be varied. Typically, the composition, based on CrO content, is within the range of from about 0.01 to about 2 percent by Weight, with amounts within the range of from about 0.1 to about 1 percent by weight being preferred. Similarly, the coating weight of the CrO deposited on the surface treated may also be varied, with coatings containing CrO within the range of from about 0.1 to about 20 milligrams per square foot being typical and coating weights containing CrO in the range of from about 1 to about 8 milligrams per square foot being preferred. It is to be appreciated that, in many instances, the concentrations used and the coating weights obtained may be higher or lower than the above values with good results being obtained.

The pH of the treating solution is generally maintained at a value of from about 0.5 to about 5.0. Preferably, the composition is formulated in such a manner as to provide a pH of from about 1 to about 3.

As with the phosphate conversion coating, the fixing or treating composition may be applied to thesurface to be treated without heating the surface, as for example, the surface temperatures being substantially at room temperature, e.g., 65-75 degrees Fahrenheit, preferably with method of application being that of misting on. The surface temperatures are preferably at last about degrees Fahrenheit, and are typically within the range of from about 200' to about 600 degrees Fahrenheit with temperatures below about 350 degrees Fahrenheit most preferred.

The treating composition may be applied to the conversion coated metal surface using various conventional application techniques, such as roller coating, immersion, flooding, spraying using various spraying techniques, with the misting techniques of application being preferred. When the composition is applied to a surface at an elevated temperature, particularly when using misting techniques, the reaction is quite rapid, and generally substantially simultaneously with its application, with the previously formed phosphate or oxalate conversion coating to form a composite, substantially dry and uniform coating on the metal surface.

As has been stated hereinbefore, the compositions and process of the present invention eliminate the necessity for rinses, and important consideration in metal treatment.

It has been found that the application of these compositions to metal surfaces may be effected over a wide range of speeds for the work traveling through the misting application zone. For example, excellent results are obtained when applying the composition to metal surfaces traveling at speeds of from about 10 to greater than 500 feet per minute. By this method, there is obtained a high speed process for providing conversion coatings on metal surfaces, which process eliminates the requirement for rinsing the conversion coating after its application. The coatings produced by the method of the present invention are found to provide excellent corrosion protection on the metal surfaces to which they are applied and also are found to be excellent paint base coatings. Additionally, they also prevent white corrosion on storage of hot dip galvanized metal.

In order that those skilled in the art may even more readily understand the present invention and the manner in which it may be practiced, the following examples are given. It is to be understood, of course, that these examples are merely illustrative of the present invention and are not to be taken as limitations thereof.

EXAMPLE 1 A Zincgrip panel (produced, ostensibly, by the continuous hot-dip zinc coating process disclosed in US 2,197,622) was solvent cleaned and heated to a temperature of about 275 degrees Fahrenheit. An aqueous solution containing 0.1 percent Zn, 0.15 percent Fe, 1 percent P and 0.05 percent HNO was applied to the panel at a PO; coating weight of about 40-50 milligrams per square foot by mist-on application. The conversion coated panel was then treated by mist-on application with an aqueous fixing or treating composition of 0.5 percent dimethylol urea and 0.2 percent CrO as having a pH of 2.1 and applied to deposit about 0.15 milligrams per square foot and CrO at about 3 milligrams per square foot. The treated panel was then painted with an alkyd-melamine baking enamel and subjected to 5 percent salt spray, humidity and physical tests. The panel, when subjected to the humidity test, which consisted of exposure in a walk-in room at 100 percent relative humidity at 100 degrees Fahrenheit for 504 hours, exhibited a few blisters, rated according to ASTM designation D714-56 on a scale of dense, medium dense, medium, few medium, few and very few. When subjected to the physical testing, adhesion being determined by knife blade and results reported on a scale of 0 to 10, where is excellent, 8 is good, 6 is fair, 4 is poor, 2 is very poor and 0 is complete loss of adhesion, the panel has a rating of 8. When subjected to a forming test, wherein the panel was subjected to a severe deformation producing parallel short radius right angle bends and paired three dimensional short radius shoulders in a single operation, the degree of failure was only 5 percent. Exposure to a 8 5 percent sodium chloride fog (ASTM test B117-61) for 504 hours when scribed according to ASTM test D-l65461 resulted in only inch creepage from the scratch.

EXAMPLE 2 The procedure of Example 1 is observed with the exception that the treating composition contains zinc dichromate as the source of hexavalent chromium. Similar excellent results are obtained as in Example 1.

EXAMPLE 3 The procedure of Example 1 is observed with the exception that the treating composition contains ammonium dichromate as the source of hexavalent chromate. Similar excellent results are obtained as in Example 1.

EXAMPLE 4 The procedure of Example 2 is observed with the exception that the fixing agent in the treating composition is hexamethylol melamine. Similar excellent results are obtained as in Example 1.

EXAMPLE 5 The procedure of Example 2 is observed with the exception that the fixing agent in the treating composition is monomethylol dimethylhydantoin. Similar excellent results are obtained as in Example 1.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is further understood that each element recited in any of the following claims is intended to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for coating aluminum, zinc and ferrous metal surfaces which comprises applying to the metal surface to be coated an oxalate or phosphate conversion coating composition, effecting formation of a substantially dry, uniform conversion coating on said metal surface, a portion of which is water-soluble; and thereafter applying to said coating containing a water-soluble portion an effective amount of an aqueous treating composition containing from 0.01 to about 2.0 percent hexavalent chromium and from 0.1 to about 6.0 percent of an organic esterifying and/or reducing component selected from the group consisting of monohydric alcohols, diols and polyols containing from 3 to 20 carbon atoms; dimethylol urea; methylol dimethyl hydantoin; alkanol amines; hexamethylol-melamine; urea-formaldehyde; hydroquinone; pyrocatechol, pyrogallol, p-methylaminophenol sulfate and N-p-hydroxyphenylglycine, thereby reacting out the water-soluble portion to produce water-insoluble phosphates or oxalates and form a substantially dry, uniform, composite coating on said metal surface.

2. A method as claimed in claim 1 wherein the conversion coating composition is an aqueous phosphate composition wherein the predominate source of phosphate ions are metal phosphates, the metal of which is selected from the group consisting of zinc, ferric iron, ferrous iron, nickel, chromium, cobalt, calcium, cadmium, manganese and mixtures thereof.

3. The method as claimed in claim 2 wherein the conversion coating composition is sprayed onto the metal surface and the metal surface is at a temperature of at least about degrees Fahrenheit.

4. The method as claimed in claim 3 wherein the conversion coating composition has a phosphate concentration of from about 0.1 to about 10 percent by weight and is applied so as to provide a phosphate coating weight of from about 10 to about 250 milligrams, and the treat- 9 ing composition is applied so as to provide a coating weight of the organic component of from about 1 to about 100 milligrams per square foot of metal surface and a coating weight of CrO of from about 0.1 to about 20 milligrams per square foot of metal surface.

5. The method as claimed in claim 1 wherein the fixing composition also contains colloidal silica.

6. The method as claimed in claim 5 wherein the colloidal silica is present in an amount of from about 0.1 to about 0.5 percent by Weight.

7. The method as claimed in claim 1 wherein the metal surface to which the conversion coating is applied is at a temperature of from about 175 to 600 degrees Fahrenheit.

8. A metallic surface having a protective coating thereon produced in accordance with the method of claim 1.

References Cited UNITED STATES PATENTS Schritfman 1486.l6 Schuster et al 148-62 X Hyams 1486.16 Somers 1486.2

Tanner 1486.l6 Ley et a1. 148-6115 McDonald 148-616 Plaxton 1486.l6

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,706,604 Dated December 19, 1972 Inventor(s) Elmer X n It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The term of this patent subsequent to April 6, 1988, has been d isclaimed.

n {'3 C sealeo time it? say of Apr-El 1:7.1.

rlttestz H v C Shall SHALL- Ds-i b: IL'TZZ C. 33501? Commissioner of Patents .ittesting Gfficer and Trademarks FORM PC4050 (10-69) USCOMM-DC 60376-P69 U.S GOVERNMENT PRINTING OFFICE 930 

